when using --enable-debug and gcc, provide the -Wno-format-nonliteral option

to prevent the warning in mprintf.c:

(currently line 930) "format not a string literal, argument types not checked"

configure.ac

@@ -1031,7 +1031,7 @@ AC_HELP_STRING([--disable-debug],[Disable debug options]),
     CPPFLAGS="$CPPFLAGS -DCURLDEBUG"
     CFLAGS="$CFLAGS -g" 
     if test "$GCC" = "yes"; then
-       CFLAGS="$CFLAGS -W -Wall -Wwrite-strings -pedantic -Wno-long-long -Wundef -Wpointer-arith -Wnested-externs"
+       CFLAGS="$CFLAGS -W -Wall -Wwrite-strings -pedantic -Wno-long-long -Wno-format-nonliteral -Wundef -Wpointer-arith -Wnested-externs"
 
        dnl here's a more aggressive set to use:
        dnl CFLAGS="$CFLAGS -W -Wall -Wwrite-strings -pedantic -Wno-long-long -Wundef -Wpointer-arith -Wnested-externs -Wcast-align -Winline -Wstrict-prototypes -Wmissing-declarations -Wmissing-prototypes -Wshadow -Wfloat-equal -Wsign-compare -Wunreachable-code"